Spectrum+pathtracer = Spectracer

A little out of curiosity test.

So it's just a brute-force backwards light path tracer, except it traces different light frequencies separately.

This is a test to get some "physically-correct" light decomposition and rainbow effects

Build

windows users may just use pre-built binaries
nix users may just do nix run github:pedohorse/spectracer, or build, then run, well, you know
(note, nixpkgs's zig hook builds with ReleaseSafe, not ReleaseFast)
to build it the most boring way:
- you need to install dependencies: embree3, libspng (maybe dev versions, depending on your package repo)
- build script expects those libs to be installed to default system location (/usr/include and /usr/lib), you may change that in build.zig if you have something special going on.
- zig build -Doptimize=ReleaseFast
- use zig-out/bin/spectracer

Usage

Check help with spectracer -h

Generally the idea is that you give it path to scene to render, and output picture. Additionally some flags to control the quality can be provided.

Scene description

When you provide a "scene" to render - you need to provide the path to a directory.

In that directory there may or may not be scene.json file

in both case you provide path the scene directory (not to any specific file in it), so you just run smth like:

spectracer path/to/scene output.png

or with some flags (flags must be before the last argument):
spectracer path/to/scene -1 10 -2 6 -d 3 output.png

Variant1: no scene.json

All files in the scene directory will be treated as obj files.

material and properties will be inferred from file name, structure is:

<name>_<material>[_<mat_param>]*[_h].obj

name can be anything
material can be one of
- lambert
- light
- glass
- mirror
further parameters depend on material.
optional _h in the end will disable tesselation for this object.
for better tesselation control use scene.json variant

For example:

foo_lambert_0.5_0.8_1.obj
means that this object will have simple diffuse shading, with color (rgb) 0.5, 0.8, 1.0
somename_light_5_5_5.obj
means that this object will be a light, with intensity (rgb) 5, 5, 5
blablabla_glass_1.3_0.1.obj
means this object is transparent and refractive (no reflections), 1.3 here is base IoR, 0.1 is IoR shift. Light of the longest wavelength (red) will be refracted with IoR 1.3, light of the shortest wavelength (purple) will be refracted with IoR 1.3+0.1=1.4 (with all other wavelenghts having IoR linearly interpolated from 1.3 to 1.4)
This is where spectrum decomposition (rainbow) happens
bibabuba_mirror
means this object is a perfect mirror.

Variant2: with scene.json

In this case, name of obj files do not matter, only scene.json file is read.

This json file is expected to contain all information about scene objects and materials. Example would be:

{
  "objects": [
    {
      "file": "bg.obj",
      "mat": {
        "lambert": {
          "cr": 0.9,
          "cb": 0.9,
          "cg": 0.9
        }
      }
    },
    {
      "file": "tube1.obj",
      "mat": {
        "light": {
          "er": 4,
          "eb": 4,
          "eg": 4
        }
      }
    },
    {
      "file": "tube2.obj",
      "mat": {
        "glass": {
        }
      },
      "tesselation": 64
    }
  ]
}